The temperature around a top ring of a gasoline engine reaches a high temperature of 200° C. or higher by combustion of fuel. In an internal combustion engine, a piston ring and a surface of a piston ring groove of a piston (hereinafter referred to as “ring groove surface”) repeatedly collide by combustion pressure at such a high temperature. At the same time, the piston ring surface and the ring groove surface slide in a circumferential direction.
The ring groove surface has projections that have been produced in lathe turning using a cutting tool and have a height of approximately 1 μm at intervals of 0.2 mm. As a material for a piston, an aluminum alloy is generally used for weight saving of an automobile body. However, an aluminum-based material has a low heat resistance, and when the temperature is higher than 200° C., the hardness thereof is reduced. The ring groove surface collides with a piston ring at such a high temperature and slides to cause a fatigue fracture of the ring groove surface. Thus, the projections come off the surface, and an active fresh surface made of an aluminum alloy appears on the ring groove surface. Dropped aluminum alloy pieces and the fresh surface of aluminum alloy appeared in the ring groove collide with the piston ring to be brought into contact with the upper and lower faces of the piston ring and thus slide. The aluminum alloy pieces cohere to the side face of the piston ring, or “aluminum cohesion” in which the piston ring body adheres to the fresh surface of aluminum alloy of the piston occurs. The aluminum cohesion continuously occurs so long as the fresh surface of aluminum alloy continues to be generated. The aluminum cohesion proceeds, and the piston ring adheres to the piston in the ring groove. As a result, the sealing performance of the piston ring is impaired. A gas sealing function as one sealing performance is impaired to cause a blowby phenomenon in which high-pressure combustion gas flows from a combustion chamber to a crank chamber. As a result, the engine output is decreased. Further, when an oil sealing function is impaired, the consumption of oil is increased. In addition, the aluminum cohesion promotes wear of the ring groove, and the sealability between the upper and lower faces of the piston ring and the ring groove surface is impaired to increase the amount of blowby.
As a method for preventing aluminum cohesion, many methods including a method in which an aluminum alloy that is a piston base material is not brought into direct contact with a piston ring, and particularly a top ring, and a method for alleviating an attack on a ring groove by a piston ring, have been conventionally proposed.
As the countermeasures of a piston, Patent Literature 1 describes a method in which a ring groove surface is subjected to an anodic oxidation treatment (anodized aluminum treatment), and micropores formed by the treatment are charged with a lubricating substance. Since a hard coating containing aluminum oxide as a main component is formed on the ring groove surface by the anodized aluminum treatment, the aluminum alloy that is a piston base material is prevented from separating, and cohesion to a piston ring is suppressed. However, there are problems in which the cost for the anodic oxidation treatment on a piston is high and aluminum oxide has poor initial running-in properties due to hardness.
On the other hand, as the countermeasures of a piston ring, Patent Literature 2 describes a method for forming, on the side face of the piston ring, a coating in which molybdenum disulfide as a solid lubricant is dispersed in polyamide or polyimide as a heat resistant resin. According to the configuration in Patent Literature 2, the solid lubricant in the coating is cleaved and worn, and accordingly the friction coefficient of the coating is decreased. Thus, the attacking properties on a ring groove are alleviated, and aluminum cohesion is suppressed.
Further, Patent Literature 3 describes a method for forming a surface coating which includes a heat resistant resin containing a copper powder on the side face of a piston ring. In Patent Literature 3, the wear resistance is imparted to the surface coating formed on the surface of the piston ring by addition of the copper powder, and accordingly the lubricity due to the heat resistant resin can be exerted over a long period of time.
Moreover, Patent Literature 4 shows that a dry coating lubricant containing a polyamideimide resin as a main component, a coating modifier for a polyamideimide resin, and hard particles of alumina or the like is formed on a sliding face of a sliding member having a streak with a predetermined surface roughness, thus the friction coefficient can be decreased while the wear resistance and adhesive property of the sliding member can be improved. Further, Patent Literature 4 describes that from the viewpoint of balance of wear resistance and wear of an opposite material, it is preferable that the hard particles be alumina and silicon nitride having a predetermined hardness.
Recently, in order to cope with the improvement of fuel efficiency, an increase in combustion pressure of an engine proceeds. As a result, the attainment temperature around a top ring is higher. In such a situation, a fatigue fracture due to a decrease in the piston strength tends to occur, and it is difficult to maintain a coating that covers a piston ring and is made of a resin over a long period of time. In Patent document 2, the solid lubricant is added as an essential component. However, the solid lubricant itself is cleaved and worn as described above, and therefore the friction coefficient of the coating is decreased. Thus, the attacking properties on a ring groove are alleviated. Therefore, the wear resistance of the coating is low, and it is difficult to maintain the coating over a long period of time and to keep the effect of preventing aluminum cohesion. The wear of the coating is attempted to be suppressed, but the amount of the solid lubricant to be added is limited, and a decrease in the friction coefficient of the coating is also limited. For this reason, the surface of the piston material having a lowered hardness at a high temperature may be roughened. Further, aluminum cohesion may occur. Since the coatings in Patent Literatures 3 and 4 are each a resin coating, the wear resistance is limited. At a high temperature, the coating may be lost due to wear or thermal decomposition, and it is difficult to maintain the effect of preventing aluminum cohesion over a long period of time.
Therefore, at present time, a piston ring that can maintain a high effect of preventing aluminum cohesion in a high output engine over a long period of time has not been achieved.